TTC21A and male infertility: Thus far, male infertility specific phenotype (MMAF) has been reported for mutations in 13 potentially axonemal genes, DNAH1, DNAH2, DNAH17, CEP135, WDR66, SPEF2, ARMC2, TTC21A, QRICH2 and CFAP genes 43, 44, 69 and 251. Expression of some of these genes is limited to the testis, however some may also have roles in motile cilia as suggested for SPEF2 [64, 67].